The present invention relates to starting of an AC motor which is already coasting. This occurrence may be the result of a short power failure where the motor has a large moment of inertia. An immediate restart is desired upon return of the power so that the motor can be reaccelerated from its coasting speed up to the operative speed.
In contrast, conventional adjustable frequency drives are designed to start with the induction motor at rest. During acceleration, the appropriate volts-per-hertz is maintained from the low frequency and the system is slowly accelerated up to desired speed. The motor slip remains small and large motor currents do not occur. However, where the motor load has a large inertia and the motor has been suddenly stopped, it continues spinning. Therefore, a restart of the motor by turning the inverter ON, for instance where the motor stoppage was due to a short outage in the power and the latter is restored, raises a problem. When the inverter is turned ON, the actual inverter frequency and the frequency required for the motor spinning speed, will not match. Therefore, the motor has a large slip. Under the circumstance, as the inverter voltage and frequencies in the starting sequence are increasing slowly, large currents will flow and the inverter will trip. One solution has been to hold the inverter output at a low value for a period of time, and and to allow the motor and load to decelerate almost to a standstill. In many applications, though, it is desirable to be able to restart the motor as soon as possible. To do this, it is necessary to find how to match the inverter frequency with the spinning speed of the motor. This has been done with information derived from a tachometer measuring the shaft speed, hence the required starting frequency for the inverter. This solution requires the use of sensors on the motor.
In contrast, the present invention does not require the addition of sensors on the motor and it enables to restart the motor drive at any frequency including standstill.